Silicon is one of the most important modern materials that has found extensive applications in areas such as photonic, electronics, sensors, medical devices, energy storage devices, and the like. In the field of electrochemical energy storage, there has been a significant increase in recent years in development activities on the use of silicon as an anode material in lithium ion batteries due to the high theoretical storage capacity for lithium. Silicon has a theoretical specific capacity of 4200 mAh/g in its fully lithiated form (i.e. Li22Si5). This is more than 10 times of the theoretical capacity 372 mAh/g of graphite, which is the most popular commercial anode material today. However, silicon can experience up to 400 percent volumetric changes during lithiation and de-lithiation and this can introduce huge stress and strain in the material. When bulk or even micrometer scale silicon is used as an anode material in a lithium ion battery, the above stresses and strain cause physical disintegration of the active material particles, and the anode made from these materials, and can thus lead to rapid capacity fade. As such, major development efforts have been directed to the use of nanometer-size silicon materials that are small enough in dimension to release the stress and strain in the material, and thus retain its ability to act as an anode material in rechargeable lithium batteries [H. K. Liu et al., J. Mater Chem., 2010, 20, 10055-10057].
At the present time, the most promising nano-sized silicon anode material, in terms of electrochemical performance and commercial potential, is one-dimensional silicon nanowires [C. K. Chan, et al, Nature Nanotech., 2008, 3, 31-35; H. K. Liu, et al, J. Mater. Chem., 2010, 20, 10055-10057; H. Wu and Y. Cui, Nano Today, 2012, 7, 414-429; U. Kasavajjula and C. Wang, A. J. Appleby, J. Power Sources, 2007, 163, 1003-1039]. When used as the anode material in lithium batteries, silicon nanowires are able to release the stress and strain by changes in diameter and length without breaking down. This allows the silicon nanowires to retain their physical integrity and preserve the electrical conductivity required as an anode material. This provides significant advantages over other silicon nanostructures [H. Wu and Y. Cui, Nano Today, 2012, 7, 414-429; C. Chan et al, Nature Nanotech., 2008, 3, 31-35; K. Q. Peng et al, Appl. Phys. Letts., 2008, 93, 033105; H. T. Nguyen, et al, J. Maters. Chem., 2012, 22, 24618-24626].
Other one-dimensional silicon nanostructures, such as nanobelts and nanoribbons, have also been made. In the present application, the term “one-dimensional nanostructure”, or 1D nanostructure, refers to those nanostructures having at least one dimension being far greater than the other two dimensions and at least one dimension being less than 100 nm. Such 1D nanostructures typically have a length in one dimension, on a micrometer scale, and typically in the range of 1 to 1000 micrometers. Nanowires have a cross section of circular or oval shape and a diameter or equivalent diameter less than 100 nm. Nanobelts or nanoribbons have a cross section resembling a rectangular or rounded rectangular shape with a thickness less than 100 nm and a width typically in a range between 10 nm and 1000 nm. In this invention, nanobelts and nanoribbons are used as synonyms regardless of the width of their cross section.
Silicon nanobelts or nanoribbons have been reported for various applications [A. Tarasov, et al., Appl. Phys. Lett., 2011, 98, Article Number: 012114; A. Baca, et al., Adv. Fund. Mater., 2007, 17, 3051-3062]. They could also be used as anode material in lithium ion batteries with similar or even improved performance as of silicon nanowires.
However, in order to fully achieve the above advantages and materialize the full commercial potential of these anodes in the lithium ion battery industry, mass production of low cost one-dimensional silicon nanostructures, and in particular silicon nanowires or nanobelts, would be desirable.
So far, silicon nanowires can be grown by bottom-up (synthesis) and top-down (fabrication) approaches. The “bottom up” approach grows silicon nanowires using various deposition techniques performed usually under vacuum conditions, while the “top down” approach produces silicon nanowires by selectively removing part of the silicon from pure wafers or particles using various etching techniques performed either under vacuum conditions or in solution. In detail, these techniques include chemical vapor deposition, sputtering, plasma deposition, laser ablation, thermal evaporation decomposition, electron-beam evaporation, supercritical vapor-liquid-solid synthesis, reactive ion etching, lithography, electrochemical dissolution, plasma etching, and metal-assisted chemical etching, etc.
Among these techniques, vapor-liquid-solid (VLS) growth and metal-assisted chemical etching (MACE) are able to offer high-quality silicon nanowires, and are extensively employed in various development work [S. Christiansen, et al, J. Appl. Phys., 2006, 100, 084323; J. D. Holmes, et al, Science, 2000, 287(5457), 1471-147, H. C. Chen, et al, US patent US20050176264; Z. P. Huang, et al, Adv. Mater., 2011, 23(2) 285-308].
In 2009, Lee, et al., patented [U.S. Pat. No. 7,638,345] a method of/for manufacturing silicon nanowires by a solid-liquid-solid process or a vapor-liquid-solid process using a porous glass template having nanopores doped with erbium or erbium precursors. Other patents have been granted to fabricating methods of silicon nanowires using MACE process [Y. X. Wu, U.S. Pat. No. 8,044,379; L. T. Canham, et al., U.S. Pat. No. 5,348,618; A. Buchine, et al., U.S. Pat. No. 8,143,143; L. T. Canham, et al., U.S. Pat. No. 5,627,382].
Similarly to the synthesis of silicon nanowires, two kinds of approaches are used for producing silicon nanobelts or nanoribbons, i.e. top-down and bottom-up. The top-down approach uses lithography followed by chemical procedures to create the nanoribbon/nanobelt from silicon wafers, which is thus able to afford a good control of morphology and crystalline orientation [A. Tarasov, et al, Appl. Phys. Lett. 2011, 98, article#012114; A. Baca, et al, Adv Funct Mater 2007, 17, 3051-3062.]. The bottom-up approach is a chemical synthesis procedure decomposing silicon compounds precursors (mainly silane) to grow nanoribbons and/or nanobelts on a substrate, by an oxide-assisted growth (OAG) or vapor-liquid-solid (VLS) mechanism, followed by removing the substrate [D. Wei and a Chen, Q. J. Phys. Chem., C 2008, 112, 15129-15133; T. Park, et al, Nanoscale Res. Lett., 2011, 6, 476; W. Shi, et al, J. Am. Chem. Soc., 2011, 123, 44, 11095-11096; N. Elfstroem and A. Karlstroem, J. Linnrost, Nano Lett., 2008, 8, 945-949; M. Huang, et al, Nano. Lett., 2009, 3, 721-727].
However, none of the above processes can be scaled up economically for mass production either because of processes complexity, including requirement of expensive production systems and process control, or because of the high raw materials cost. For example, the VLS process involves preparation of thin catalyst film (usually precious metals, e.g., gold or platinum) onto a silicon wafer substrate by sputtering deposition or thermal evaporation, liquidizing the catalysts at high temperature and injection of silane gas (highly reactive and toxic). In such a process, approximately 85% the injected silane gas leaves the reaction system unreacted, which results in a low yield and also requiring complex off gas handling. On the other hand, although MACE process proceeds in liquid phase without emitting toxic silane, it employs high-cost silicon wafer and noble metal salts (e.g., AgNO3), which significantly increases the overall production cost. Furthermore, the silicon nanowires from these processes are usually not uniform in size because of the difficulty in controlling the size of the noble metal particles. Also, the etching does not go through the wafers, and most of the residual wafer silicon is left unutilized.
Some effort was made to produce three dimensional porous silicon powders with nanopores created by chemical etching or de-alloying process. For example, U.S. Pat. No. 7,569,202 B2 disclosed a process producing silicon nanosponge particles by etching metallurgical silicon particles. US patent publication No. 2004/0214085A1 disclosed another process to make similar silicon porous particles by de-alloying nickel from quenched (high rate cooling) silicon-nickel alloy particles. None of the above work was able to produce one-dimensional silicon nanostructures because there is no one-dimensional or two-dimensional silicon nanostructures existed in the microstructure of the metallurgical silicon or the quenched silicon-nickel alloy.
Economic, mass production of one-dimensional silicon nanostructures including silicon nanowires or nanobelts remains one of the biggest challenges in their commercial applications. Accordingly, a principal advantage of the present invention is the provision of a method which facilitates the economical production of silicon nanowires and/or nanobelts.